Drawing ruler lines

Hi!

I need to draw ruler lines (parallel to axes) intersecting
the mouse pointer position. That is:
          
         y> >

···

--------------*--------------

          > >
          > >
          > >
          > >
          > >
          +-----------------------------
                                       x

I'm not sure where is the best place to do such a thing:
if at renderer or at artist level. The rulers
must follow the mouse and everytime they are
moved the contents "below" them must be restored.
I don't know how to do it (to save the underlying
contents before rendering the ruler). Another matter
is tranformation between data and screen coordinates,
how could I do it? I played a little
with events, line artist and with the renderer api but
I would welcome any clue from you.

Thank you in advance.
Regards,
Carlos

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